1. Field of the Invention
The present invention relates generally to hollow struts in jet propulsion engines, and more particularly, to a damper assembly for a strut in a jet propulsion engine.
2. Description of Related Art
Jet propulsion engines include a family of engines known as "transonic" jet propulsion engines. These transonic jet propulsion engines may be of a turbofan type capable of operating at transonic or supersonic speeds. The transonic jet propulsion engines typically include a front frame, the upstream end of which forms an inlet sized to provide a predetermined airflow, and a fan directly behind the front frame for pressurizing an inlet airflow. Downstream of the fan is a core engine for combusting fuel mixed with the pressurized air to produce combustion gases which are discharged to obtain a propulsive force for the engine.
The front frame typically includes a cast outer cylindrical case or shroud, an inner circumferential support or hub ring, and a plurality of circumferentially spaced apart and radially outwardly extending fixed struts disposed between the outer cylindrical case and the inner circumferential hub ring. An internal strut stiffener is generally disposed between the walls of the strut to resist buckling of the strut walls.
The fan typically includes a fan rotor which rotates a plurality of blade assemblies in at least one or more rows or stages. During assembly or operation of the fan, physical variations may exist in or between the blade assemblies. For example, variations may exist as to the spacing of the blade assemblies circumferentially about the rotor or as to the leading edges of the blade assemblies, e.g. nicked or blunt.
When the fan blades are operated at transonic or supersonic speeds, these physical variations in the first stage blade assemblies of the fan will produce air stream pressure pulses or fluctuations known as "multiple pure tones". These multiple pure tones travel forward and excite the strut or vibrate the strut at its natural frequencies. This occurs over a broad range of frequencies.
One disadvantage of the above arrangement is that high cycle fatigue may cause cracking of the struts. The cracking occurs as a result of excitation of underdamped first flexural and torsional strut natural frequencies due to the multiple pure tones. Another disadvantage is costly repair of the struts due to cracking. One advantage that the present invention provides is a cost effective repair.